Long wavelength infrared detector with heterojunction

ABSTRACT

An infrared radiation detector having a first semiconductor layer depositedn a substrate to form a diode junction with an overlay contact, is rendered more effective to detect long wavelength radiation by deposit of a second semiconductor layer between the first layer and the overlay contact in a heterojunction arrangement. The semiconductor materials are selected so as to separate radiation absorbing and electrical functions respectively performed within the two layers and to produce an enhanced output across the diode junction between the first layer and the overlay contact.

BACKGROUND OF THE INVENTION

This invention relates generally to radiation detectors and inparticular to infrared detectors of the photovoltaic diode type asdisclosed in prior copending application Ser. No. 07/539,958, filed June18, 1990 and now U.S. Pat. No. 5,012,083, with respect to which thepresent application is a division.

Photovoltaic diode detectors presently available usually have limitedwavelength signal detection capabilities because of bandgapcharacteristics of its semiconductor material in a photon-counter modeof operation. Because of the narrow bandgap of the single semiconductorlayer involved, diode junction resistance, which is mathematicallyrelated to bandgap magnitude, is correspondingly small when longwavelength radiation is being sensed. The signal voltage output of suchphotodiode is also small as a result of the relationship of thewavelength to the bandgap of the semiconductor material. The smallbandgap involved also gives rise to a high noise level in the signaloutput even in the absence of illumination because of the backgroundcomponents of the radiation being absorbed causing generation of chargecarriers (conduction band electrons and valence band holes) transportedacross the diode junction to create random noise in the process. Oneexample of such a photodiode detector of a single semiconductor layertype is disclosed in U.S. Pat. No. 4,763,176 to Ito.

Heterojunction types of multi-layer photodiode detectors on the otherhand are disclosed for example in U.S. Pat. Nos. 4,297,717, 4,686,550and 4,763,176 to Li, Capasso et al. and Ito, respectively. However, noneof the latter three patents are specifically concerned with enhancingsignal outputs heretofore degraded when detection of relatively longwavelength infrared radiation is involved.

Accordingly, it is an important object of the present invention toprovide an enhanced signal from a photodiode type of infrared radiationdetector having a small bandgap semiconductor layer through which longwavelength radiation is absorbed.

An additional object in accordance with the foregoing object is toenable a greater flexibility in the design of photodiode detectors tomeet different requirements with unexpectedly high signal outputvoltages and a reduced signal-to-noise ratio in response to detection ofinfrared radiation within a relatively long wavelength band.

SUMMARY OF THE INVENTION

In accordance with the present invention, the problems and operationalwavelength limits associated with photodiode detectors of theaforementioned type, are circumvented by deposit on a first narrowbandgap layer having a radiation absorbing function, a second widerbandgap layer of semiconductor material limited in function toelectrical signal enhancement with substantially no radiationabsorption. The position of the conduction band edge of such secondsemiconductor layer is interrelated with that of the radiation absorbinglayer on which it is deposited so as to substantially overcome anypotential barrier to electron movement toward the diode rectifyingjunction formed with an overlay on the second semiconductor layer withincreased junction resistance. The valence band edge of the second layeris furthermore lower than the valence band edge of the first narrowbandgap layer supported on the detector substrate to form a barrieragainst hole movement toward the junction while the thickness of thesecond layer is such as to both accommodate band bending development andavoid appreciable radiation absorption.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

Other objects, advantages and novel features of the invention willbecome apparent from the following detailed description of the inventionwhen considered in conjunction with the accompanying drawing wherein:

FIG. 1 is a partial side section view through a photovoltaicheterojunction diode type of infrared radiation detector constructed inaccordance with one embodiment of the present invention.

FIG. 2 is an energy band diagram corresponding to the heterojunctionstructure depicted in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawing in detail, FIG. 1 depicts a heterostructuretype of photovoltaic detector, generally referred to by referencenumeral 10, having a transparent substrate 12 on which an activeinfrared radiation absorbing semiconductor layer 14 is deposited. Thelayer 14 is adapted to be exposed through substrate 12 to infraredradiation 16 within a relatively long wavelength band, such as 8 to 12μm. A second relatively thin semiconductor layer 18 is deposited on thelayer 14. The layer 18 has a thickness of 0.2 μm, for example, toestablish a current-resistance drop across a junction formed between anoverlay 20 and the layer 14 to which electrical leads 22 and 24 may beattached as shown. According to one embodiment, the overlay 20 is madeof an appropriate metal forming a Schottky type diode junction withlayer 14 across which a signal output voltage 26 appears as sensedthrough lead lines 22 and 24 for radiation detection purposes as shown.Such output voltage occurs in response to photons of energy in theimpinging radiation 16 being detected which is higher than the bandgapof semiconductor layer 14 and results from electron-hole pairs generatedunder the conditions indicated.

The interface 28 between the first semiconductor layer 14 and thetransparent substrate 12 establishes the limit of a relatively narrowbandgap energy region 30 as diagrammed in FIG. 2 between conduction andvalence band edges 32 and 34 establishing the energy bands for theheterojunction structure. Such energy bands are determined by selectionof bandgap modifying impurity compounds for the semiconductor materialsof layers 14 and 18, including the establishment of a wide bandgapenergy region 38 extending from the interface 36 between layers 14 and18 and the overlay interface 40. The bandgap of region 30 is less than0.15 eV while the bandgap for region 38 is approximately equal to orhigher than 0.40 eV according to one embodiment of the invention.

As a result of the foregoing heterojunction arrangement for photovoltaicdetector 10 and the aforementioned selection of semiconductor materialsfor its layers 14 and 18, mismatch at the heterojunction interface 36between layers 14 and 18 and band bending of the Schottky junctionoccurs, as diagrammed in FIG. 2. Such conditions are associated with thegeneration of electric fields having the current resistance dropaforementioned separating electrons and holes in connection withotherwise conventional operation of photovoltaic detectors. Also,because of the establishment of a higher diode junction resistancebetween layer 14 and overlay 20 resulting from the larger bandgap energyof layer 18, the signal output 26 sensed is enhanced while radiationabsorption and electrical functions, to which the layers 14 and 18 arerespectively limited, are separated.

With respect to the aforementioned selection of semiconductor materialsfor the layers 14 and 18, certain requirements are imposed on theinterrelationships between the operational properties of thesemiconductor materials in accordance with the present invention. First,the conduction band edge 33 for layer 18 must be equal to or lower thanthe conduction band edge 32 of layer 14 according to one embodiment inorder to eliminate therein the potential barrier to electron movementtoward the Schottky junction during charge transport. Also, the valenceband edge 37 of layer 18 should be lower than the valence band edge 34of layer 14 to or block establish a barrier to hole movement toward theSchottky junction during charge transport. The thickness of the layer 18as hereinbefore specified is thin enough to substantially avoid opticalsignal absorption and yet sufficient to accommodate development of bandbending as diagrammed in FIG. 2.

In order to meet the requirements and band alignment conditions ashereinbefore specified, the semiconductor materials for layers 14 and 18may be of the p-type preferably selected from the same class or familyof compounds within groups II, IV and VI with Pb, Cd and Sn as cationsand S, Se and Te as anions or within groups II and VI with Cd, Zn and Hgas cations and Te and Se as anions. In general, the semiconductormaterial selections for the layers 14 and 18 should becrystalallographically and chemically compatible.

In an alternative embodiment, the Schottky type metal overlay 20 may bereplaced by a n-type overlay to establish a p-n junction through layer18. According to yet another alternative, the second layer 18 may beprovided with a conduction band edge slightly higher than that of layer14 by no more than approximately 15 meV. In the latter case, thermalexcitation should overcome any barrier to electron migration toward thejunction formed with overlay 20 without excessive signal degradationunder environments above liquid nitrogen temperatures.

The present invention contemplates yet other variations wherein thesemiconductor materials for layers 14 and 18 are of the n-type ratherthan the p-type. In such case, the positions of the layer conductionband edges and valence band edges as hereinbefore described areinterchanged.

Still other modifications and variations of the present invention arepossible in light of the foregoing teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:
 1. A method of detecting radiation by generation ofconduction band electrons and valence band holes, comprising the stepsof: providing a first semiconducting region having an energy band formedby a conduction band material; providing a second semiconducting regionformed by a material establishing a current-resistance drop; generatingan electric field extending through said regions with saidcurrent-resistance drop; exposing said regions to the radiation beingdetected to generate said conduction band electrons and the valence bandholes; and sensing electrical voltage appearing across said regions as aresult of said generation of the conduction band electrons and thevalence band holes.
 2. The method of claim 1 wherein said materials fromwhich the regions are formed respectively eliminate potential barrier toelectron movement and establish a barrier to hole movement.
 3. Themethod of claim 2 wherein said materials are selected bandgap modifyingcompounds.
 4. The method of claim 3 wherein said step of providing thesecond semiconducting region includes: selection of the bandgapmodifying compound thereof to increase resistance while limitingradiation absorption to the first region.
 5. The method of claim 1wherein said step of providing the second semiconducting regionincludes: selection of the material thereof to increase resistance whilelimiting radiation absorption to the first region.
 6. A method ofdetecting radiation by generation of conduction band electrons andvalence band holes, comprising the steps of: providing a firstsemiconducting region having an energy band established by a firstmaterial eliminating potential barrier to electron movement; providing asecond semiconducting region as a heterojunction formed by a secondmaterial in a class having properties interrelated with those of thefirst material while substantially limiting radiation absorption to thefirst region and increasing resistance; generating an electric fieldextending through the regions under said increased resistance; exposingsaid regions to the radiation being detected to generate said conductionband electrons and the valence band holes; and sensing electricalvoltage appearing across said regions as a result of said generation ofthe conduction band electrons and the valence band holes.
 7. In a methodof detecting incident radiation, the steps of: providing a semiconductordevice having layers of semiconducting material; modifying thesemiconducting material within the respective layers by differentselections of impurity compounds establishing relatively narrow and wideband-gap regions; exposing one of the layers to the incident radiationfor absorption thereof within the narrow band-gap region; and disposinganother of the layers proximate to said one of the layers to limitdetection of the incident radiation by the layers by absorption ofphotons in a long infrared wavelength spectrum.
 8. The method of claim 7wherein said other of the layers establishes the wide band-gap regionwithin which substantially no absorption of the incident radiationoccurs.
 9. A method of detecting incident radiation, comprising thesteps of: providing a layered semiconductor device having layers ofsemiconducting material with different amounts of band-gap modifyingcompounds therein to establish an IR active zone; receiving incidentradiation of said layered semiconductor device; absorbing said incidentradiation in the layered semiconductor device; generating longwavelength infrared photons as a result of absorbing said incidentradiation; and detecting said photons generated in said semiconductordevice.
 10. The method of claim 9 wherein the step of detecting saidphotons includes: absorbing said photons in said IR active zone withinone of the layers of the device.